The present invention relates to a packaged semiconductor device structure in which a semiconductor chip including semiconductor devices such as those having a memory function is packaged in a sealed fashion.
Usually, a semiconductor chip is sealed in a package made of a ceramic, glass or plastic (resin).
It is known that when a semiconductor chip including a memory circuit such as a CCD (charge coupled device) memory circuit, a one-MOS dynamic memory circuit or a static memory circuit is sealed in such a package, malfunction of the memory circuit occurs because of adverse effects of the package. More particularly, the package emits alpha (.alpha.) particles or alpha (.alpha.)-rays which cause the memory circuit to malfunction.
It has been proposed in U.S. patent application Ser. No. 92,502 (filed on Nov. 8, 1979 and corresponding to DOS No. 2946801 laid open on May 22, 1980) filed on the basis of Japanese Patent Application No. 142375/78 in the name of Hitachi, Ltd., to coat the surface of the semiconductor chip to be sealed in a package with a film of an organic material which emits less alpha particles, such as a polyimide resin in order to prevent the malfunction of the memory circuit due to the alpha particles.
The inventors of the present invention have found that the packaged semiconductor device structure processed in the manner described above has the following disadvantages.
The organic material film is decomposed to produce moisture and/or gas such as CO.sub.2 gas at a temperature which occurs when the semiconductor chip is packaged. The moisture especially erodes a low resistance aluminum metalization layer formed inexpensively on the semiconductor chip. The polyimide organic material film can endure temperatures up to approximately 380.degree. C. Accordingly, when the semiconductor chip is packaged at a temperature higher than 380.degree. C., the organic material film will be decomposed and the problem discussed above is encountered.
In order to package the semiconductor chip at a packaging temperature lower than 380.degree. C., sealing with a Au-Sn eutetic alloy brazing material or seam welding may be used. However, when such a sealing method is to be used, sealing portions of the packaged structure must have metallic material. More particularly, a laminated ceramic package is required to have ceramic package members having a metallic material coupled or adhered to their joining or sealing surfaces. Further, it has been found that with such a laminated ceramic package (1) the cost of packaging material is one order of magnitude more expensive than that of other packaging material, and (2) working cost for mounting the semiconductor chip in the package is high.
As an approach to reduction of the packaging cost, use of a structure sealed with a low melting point glass (including no less than 60% by weight PbO) may be considered. In this method, less expensive glass than Au-Sn used in the laminated ceramic pacakge can be used, and the metallization of the sealing portion of the package is not required. Accordingly, the cost can be substantially reduced. However, the glass melts at a temperature of approximately 400.degree.-450.degree. C. The sealing temperature, therefore, should be about 410.degree.-460.degree. C. As a result, in spite of the advantage of cost reduction, this method may degrade the device performance by the erosion of the aluminum metallization by the decomposition of the organic material coating on the semiconductor chip as discussed above.
The inventors have further found that the use of a low melting point glass for sealing or joining the package members requiring such a high sealing temperature as mentioned above causes a substantial increase of the gas pressure within the package due to the above-mentioned gases produced by the organic material film on a semiconductor chip, and that the low melting point glass is very likely to be deformed by the gas pressure increase thereby to reduce the mechanical strength of the sealing.